As shown in FIG. 4, a prior-art power steering mechanism is provided with an input shaft 101 rotatable in response to the turning of a steering wheel (not-shown), an output shaft 105 connected by a torsion bar 103 to the input shaft 101, and a control valve 107 composed of valve members provided on the peripheries of the input and output shafts 101 and 105. When the input shaft 101 is rotated, the control valve 107 is operated to control the supply and discharge of pressurized hydraulic fluid. In the prior-art power steering mechanism, as shown in FIG. 4, the lower end 103a of the torsion bar 103 is an interference fit into the output shaft 105, while the upper end 103b of the torsion bar 103 is inserted into the input shaft 101. By driving a pin 115 into holes 111 and 113 formed in the input shaft 101 and the upper end 103b of the torsion bar 103, respectively, the input shaft 101 and the torsion bar 103 are fastened together.
The precision required in processing the pin holes 111 and 113 is difficult to attain. When the holes 111 and 113 are bored and the pin 115 is driven into the holes 111,113, the input shaft 101 and the torsion bar 103 are easily displaced in the rotational direction due to imprecise machining of the pin holes 111 and 113. Since the input shaft 101 and the torsion bar 103 are difficult to secure in a desired relative position, unsatisfactory articles easily result from the prior art.